This invention relates to a compressor, of a type which can be incorporated in an exhaust-gas turbocharger, comprising a compressor housing enclosing a compressor wheel and fastened, by its rear wall, to a stator housing mounting a drive shaft, and to a method of establishing a damping within the compressor housing rear wall.
Known high-speed compressors, more especially of the type incorporated in exhaust-gas turbochargers, produce considerable noise because the material or materials from which the compressor is made allow easy passage therethrough of any mechanical vibrations produced by the compressor during its operation. The vibrations causing the noise are produced, for example, by the rotation of the rotor when it is in imbalance. These mechanical vibrations (or oscillations) are passed on by way of the stator components and are radiated at the compressor housing. In this respect, resonance phenomena arise at the compressor housing and the rear wall thereof.
The level of this noise, which can be very burdensome when it is in the high frequency region, is extremely difficult to reduce. One solution for reducing the level of noise, which has been used with some success, is the very fine balancing of the rotor. This solution has proved, however, extremely expensive to adopt. Moreover, the center of balance of the rotor shifts during its operation, so that any noise reduction can only be achieved for a short time. The main cause of the increase in the level of noise during operation of the compressor is because of newly-arising imbalances at the rotor as a result of wear and ageing, and also thermal distortions or elastic-plastic deformations of the rotor as a result of mechanical stresses due to centrifugal forces, as well as internal moments of the bend-soft rotor.
The possibility does exist of improving the damping of the conduction of sound through the compressor between rotor and stator by an enlargement of the damping gap of the friction bearings, but this solution has only a slight value, since in the interests of a smooth shaft run, i.e. an optimizing of the bearing-rotor system from the rotor-dynamic point of view, changes in the size of the bearing gap are scarcely permissible.
Finally, the level of noise produced by the compressor can also be affected by a difference in the mass ratios between the compressor housing in the compressor housing's rear wall. However, this latter solution results in a compressor of considerably higher weight, in which respect hindrances of other functions occur, and an economical mass production of the compressor is hardly possible.
The mounting of the separating ring directly on the two portions of the compressor housing rear wall may be complicated for the production, in particular because an optimum material for the separating ring may not form a good bond with the parts of the compressor housing rear wall which is normally made from an aluminium diecasting so that the detachment phenomena by shrinkage of the separating ring material after the penetration are not avoided with certainty.